Southern California braces for another September heat wave with temperatures projected at near record levels, with temperatures already set to reach into the 80s inland and 90s across much of the south.
The heat will be felt from San Diego in the north to Riverside in the south, with temperatures forecast to be even higher in the southern and northern mountains.
The National Weather Service forecasts that the heat wave will start Tuesday night and last until Wednesday night.
While temperatures are likely to climb into the 100s inland on Wednesday, and the 90s across most of the western U.S. and the Pacific Northwest, they will be the most comfortable midweek temperatures seen in Southern California since May 2016 when the hottest month on record was recorded in September.
Related: What you need to know about the ‘Southern California heat wave’
The heat wave is caused by a combination of three factors, the NWS says: an enhanced Pacific Decadal Oscillation, a strong La Niña and a high-pressure system that has settled across the West.
Here is the summer weather forecast for September for the U.S. and Southern California:
National Weather Service
National Weather Service/Locations
The NWS forecast shows a high of 88 on Thursday and a low of 81 on Friday in San Diego, before starting the expected heat wave on Tuesday night.
“It’s going to be one of the biggest heat waves in Southern California history,” said NWS meteorologist Joe Fortunato.
“It will spread from San Diego to Ventura County and up into the interior.”
The heat wave will be a major concern for some local communities, such as the 10 percent of San Diego County that are at risk of extreme heat as a result of a heat wave forecast by the NWS.
There are also impacts from the heat wave and other weather fronts on people out of state, Fortunato said: “Those areas will have the risk of heat index values of 92 or more out of the 90s.”
He expected the hot weather to begin in the early morning hours Tuesday on the East Coast, then move north on Wednesday.
“It’s going to be hard for people to sleep, that’s